Vibrating rollers are generally used for the compaction of construction and surfacing materials for roads, airfields, car parks, etc. In practice, obtaining the best compaction results for different types of surfaces requires the application of different vibrational characteristics, such as low frequency, large amplitude vibrations for compacting soil, and high frequency, small amplitude vibrations for asphalt. A vibrating roller is therefore more efficiently utilized when it is capable of varying vibrational characteristics.
The vibrational characteristics of a roller may be varied by means of adjustable eccentric elements. In one known device, a volume of liquid held in a cavity positioned eccentrically on a rotating shaft acts as an eccentric weight. The cavity is connected to a hydraulic cylinder which supplies or removes liquid under pressure into or from the cavity. Adjusting the amplitude of the eccentric moment, i.e. the deviation of the moment of inertia of the eccentrically placed element with respect to the rotational axis, is thus accomplished hydraulically, independently of the direction of rotation or the rotational frequency.
However, this known device is disadvantaged by its relatively complicated construction and also requires a special drive system to operate the hydraulic cylinder. In addition, moving fluid under pressure from the cylinder into the eccentrically situated cavity takes time, and therefore a change of amplitude cannot be accomplished quickly. Further, the problem of leakage in hydraulic systems of this type results in further complications and less accuracy in setting the required amplitude.
Another known device utilizes spring-loaded eccentric weights on a rotating shaft which at different rotational speeds assume different angular positions in relation to each other and thereby give rise to a change in the resulting vibrational amplitude. The disadvantage with this device, however, is that, from a configuration producing high frequency and low amplitude, the device cannot be stopped without passing through a configuration of high amplitude, if it is desired to change the direction of shaft rotation, for example.